Accuracy and heat was Re: re: Re: New file upld to CCED

Smoke wrote:

>I think what I'd probably do to overcome the problem of heat
>expansion on these shafts is make the bearing mount on the end where
>the motor drive connects fixed so it CANNOT move axially. Then on
>the other end, I'd let the shaft float axially in the bearing
>mount. The amount of expected expansion can be calculated and
>allowed for.

Yes. That's EXACTLY what is commonly done. (Compensation via S/W)

>So, with the ball nut connected to a moving part via some sort of
>bracket, I see no reason to worry about expansion. The electrical
>system should be set up to ALWAYS "register" from some point near
>the fixed end of the shaft. That way any expansion would always be
>cumulative.

But this is exactly WHY you DO need to consider expansion! And below
you QUANTIFY the problem.

>Let's consider how much expansion there's going to be:
>With a coefficient of expansion of .00000633 inches per inch per
>degree F, and a 50 degree temperature rise, a 48" shaft will
>expand a total of .01519".

Here's the meat of the issue, IMO. You've given .015 in 4 feet; this
is either a LOT, or INSIGNIFICANT, depending on the app! Art Fenerty
routing signs is not going to even notice this; But some of the
machinists out there may find this unacceptable.

For MOST, it is NOT enough to matter!!!

>This means a shaft starting out at 68 degrees would heat up to 118
>degrees during operation. With a 25 degree temperature rise,
>there would still be .0075" expansion.

>This, of course assumes the shaft temperature rise was evenly
>distributed over the entire length of the shaft.

Another KEY point. It probably isn't :-(

>So the builder will have to consider how much temperature rise is
>permissible during operation AND allow for expansion OR provide a
>method of cooling the ball nut or screw...or both. Based on the
>amount of expected expansion over 48", I'm sure I'd try to get the
>unit to run as cool as possible, by keeping preload to a bare
>minumum.

>Does this help?

Smoke,

Excellent post! The trick here is knowing when the numbers can be
ignored, and when they must be respected. You didn't get into the
effects of DIFFERENTIAL expansion (steel shaft mounted on aluminum
structure) or large mass/small mass temp. rise co's. And we put the
machine in a garage where the temp is constantly changing...

Many beginners will continue to think they can get .0001 because
that's their step size. We know better :-)

Ballendo